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Energy-efficient A/C knows when you’re in the room. By Dave Mabe

When I began working from home, I converted a shed in the backyard into an office. The shed wasn’t heated or cooled, and summer was bearing down, so I needed to find an HVAC solution quickly. I didn’t want to extend my home’s HVAC system because this would have been lot of trouble, quite costly, and wasteful; I knew the office would remain unoccupied much of the time. Being a home automation enthusiast, I saw this as a perfect opportunity, and I came up with a cheap, efficient solution that sets the temperature to different levels depending on whether I’m in the office.

My office is about 20 feet square, so all I needed for cooling was one energy-efficient window-unit air conditioner. As with the main house, I decided to use X10 home automation, which transmits through home electrical wiring. X10 hardware is easy to install: you just plug the controller modules

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into the wall and then plug your appliances’ power cords into the modules.

The peripheral components in my system were the X10 appliance module on the A/C unit, an X10-readable thermometer, and an X10 motion sensor. The thermometer I used was part of a multi-function weather station that takes indoor temperature as one of its readings, but you just need any cheap digital thermometer that can communicate over X10. To control the system, I used the excellent home automation program MisterHouse and a serial adapter that translates between MisterHouse messages on the computer and X10 signals on the power lines. (My weather station connects to a different computer in my network from the one that runs MisterHouse, but you can also plug the thermometer directly into the MisterHouse computer if it has a spare port.)

Photograph by Dave Mabe